Đáp án:
` S = {2020} `
Lời giải chi tiết:
` \frac{2-x}{2018} - 1 = \frac{1-x}{2019} - \frac{x}{2020} `
` <=> \frac{2-x}{2018} - 1 + 2 = \frac{1-x}{2019} + 2 - \frac{x}{2020} `
` <=> \frac{2-x}{2018} + 1 = \frac{1-x}{2019} + 1 + 1 - \frac{x}{2020} `
` <=> \frac{2020-x}{2018} = \frac{2020-x}{2019} + \frac{2020-x}{2020} `
` <=> (2020-x)(1/2018 - 1/2019 - 1/2020) = 0 `
` <=> 2020 - x = 0 `
` <=> x = 2020 `
Vậy ` S = {2020} `
